The cell membrane is composed of a lipid bilayer having integral proteins and forms the outer-most layer of a cell. One crucial feature of the membrane is its selective permeability. The ability of various compounds and molecules to enter and exit the cell depends upon the size, charge and other chemical properties of the molecule. If a particle is too large, or its properties incompatible with diffusion across the membrane, the particle must enter through another route, such as a protein channel or the molecule could enter through endocytosis. These alternative routes nevertheless remain as significant limitations to entry.
A number of methods have been developed to facilitate the delivery of compounds across the cell membrane and into the cell. Among the chemical methods, calcium phosphate has been used for many years to introduce DNA into a cell. In performing the procedure, a precipitate of calcium phosphate is formed, which binds to the DNA in the precipitate solution. When a suspension of the solution is added to the cells to be transfected, the cells take up the precipitate, and with it the bound nucleic acid. Liposome-based transfection has emerged in recent years as a widely practiced method of delivering foreign compounds to the interior of a cell. Liposomes are spherical vesicles that can be used to deliver compounds, such as drugs or nucleic acids, to the interior of a cell. In delivering the compound, the liposome fuses with cell membrane, allowing the content of the vesicle to gain access to the cell's interior.
An additional technique for delivering molecules to the interior of a cell is electroporation. Electroporation is performed by exposing the cells to an electric potential that traverses the cell membrane. While its mechanism is not fully understood, electroporation is believed to involve the breakdown of the cell membrane lipid bilayer leading to the formation of transient or permanent pores in the membrane that permit the chemical species to enter the cell by diffusion. The electric potential is typically applied in pulses, and whether the pore formation is reversible or irreversible depends on such parameters as the amplitude, length, shape and repetition rate of the pulses, in addition to the type and development stage of the cell. Electroporation has many advantages, including simplicity and effectiveness across a wide variety of cells. Additionally, it can be used in conjunction with other techniques to improve the efficiency of those techniques.